1. Field of the Invention
The present invention relates to a belt driving apparatus including a driving pulley, a driven pulley, and an endless toothed belt wound around the pulleys. In particular, the present invention relates to a belt driving apparatus for use in a transfer robot having a belt-driving-type linear transfer mechanism.
2. Description of the Related Art
As a type of robot for transferring workplaces, robots having a mechanism for moving a hand along a linear movement path (linear movement mechanism) are known (see JP-A-2008-272847, for example). Such a transfer robot having a linear movement mechanism has a simple structure and is inexpensive as compared with articulated robots, and hence, is widely used to transfer a wafer, a glass substrate or the like into or out of a chamber in manufacturing a semiconductor device or a liquid crystal panel, for example.
The transfer robot disclosed in JP-A-2008-272847 includes a belt driving apparatus for driving the linear movement mechanism. As shown in FIG. 11 and so on of this document, the belt driving apparatus includes a plurality of pulleys including a driving pulley rotated by a driving force from a driver source, and two driven pulleys. An endless output belt is wound around the pulleys. The pulleys are rotatable about respective rotation axes which are set generally parallel to each other. The output belt is provided with a connection member connected to a hand. The hand is slidably held by a linear guide rail and moves linearly in accordance with reciprocal movement of the connection member along a movement path between the driven pulleys. Between the driving pulley and each of the driven pulleys is provided an idler pulley held in contact with the cuter surface of the output belt for applying tension to the output belt.
In recent years, the size of a workpiece such as a panel for making a liquid crystal panel is being increased, and hence there is a demand for increasing the movement distance of a hand of a transfer robot for holding a workplace. Moreover, to enhance the productivity, transferring workplaces at high speed and with high accuracy is also demanded. In a transfer robot using the above-described belt driving apparatus, the movement distance of the hand can foe increased relatively easily by increasing the lengths of the output belt and the guide rail for supporting the hand. Further, the transfer speed can also be increased by using toothed pulleys as the driving pulley and driven pulleys and using, as the output belt, a tiding belt (toothed belt) having teeth on the inner surface for meshing with the pulleys. This arrangement prevents slipping between the output belt and the driving pulley or driven pulleys in the travel direction of the belt.
In a belt driving apparatus having an output belt wound around a plurality of pulleys, it is desirable that the rotation axes of the pulleys are set precisely parallel to each other. In practice, however, due to errors in making or mounting each pulley, it is difficult to set the rotation axes of the pulleys parallel to each other with sufficient accuracy. Further, output belts vary from each other. Because of these reasons, when an output belt runs, the output belt may deviate toward one side in the axial direction of the pulley (driving pulley or driven pulley) so that an edge of she output belt may come into contact with a collar portion provided at each end of the pulley in the axial direction. In such a case, the output belt cannot run stably and may break if continues traveling. When the length of the output belt is increased or the travel speed of the belt is increased to realize high-speed transfer of workplaces, the amount of displacement of the output belt in the width direction increases, which increases the possibility that the above-described problem will occur.